Components may be coupled to housings to form a connection and a hermetic seal. The housings may be made from a variety of materials having coefficients of thermal expansion (CTE) ranging from kovar (CTE=5 ppm/° C.) to aluminum (CTE=23 ppm/° C.). Components with a CTE less than or equal to the housing can be effectively joined and produce a hermetic seal with soft solder since this eliminates or limits solder tensile strain to acceptable limits during cool down from the soldering temperature. However, when the component with a CTE greater than the housing, for example a steel component (CTE=15 ppm/° C.), is soldered to a kovar housing, an effective solder joint is difficult as the strain in the joint upon cool down may exceed the failure strain of the solder. The use of hard solder, for example 80/20 Au/Sn, produces extremely high tensile stresses in the vicinity of the ultimate strength of the solder (40,000 psi) with low CTE housing materials. These stresses can produce at least two problems. First, the stresses can be passed on, in part, to the metal-to-glass seal interface and lead to seal failure (i.e. glass-to-metal separation or glass cracking). Second, the stresses can also produce failed solder joints particularly when any stress concentrator is present due to geometric factors or other external stresses applied during product processing or field use.
FIGS. 1–4 show a prior art method of joining an electrical component 100 to a housing 110. The component 100 has an electrical conductor 102 extending from at least one end. The electrical conductor 102 is electrically insulated from a component body 104 of the component 100 by an insulator 106. The component 100 is coupled to the housing 110 with solder that fills a gap 108 between an exterior surface 112 of the component 100 and an interior surface 114 of a hole formed in the housing 110. The housing 110 is often made from a material (typically kovar) having a similar CTE as the component 100 (also typically kovar) that is coupled thereto. The component 100 and the housing 110 may be coupled with a soft solder, for example, Sn 62, Sn 63, Sn 96, or Sb 5; or a hard solder, for example, 80/20 Au/Sn. Soft solders are feasible structurally for many applications but are less desirable for other applications such as connectors where cables may provide high stress that may lead to eventual failure. Hard solders provide necessary structural strength, but differences between the CTE of the housing 110 and component 100 (kovar, CTE=5 ppm/° C.) and the solder (CTE=15 ppm/° C.) can produce critically high stress levels in the solder joint. This tensile stress is imparted into the hard solder joint upon formation and remains in the solder joint for the life of the product due to the non-creep nature of the hard solder.
Steel is a common material for the component 150 for cost and performance reasons. In this configuration, a satisfactory solder joint may be formed when the component 150 is soldered to a housing 152 having a CTE greater than or equal to the CTE of the component 150, but an unsatisfactory solder joint is formed when the component 150 is soldered into a housing 152 having a CTE less than the component 150, since strain in the solder can be intolerable upon cool down. For example, a satisfactory solder joint is formed between a steel component and a steel, stainless steel, aluminum, copper, or brass housing and an unsatisfactory solder joint is formed between a steel component and a kovar housing (CTE=5 ppm/° C.).
Therefore, there is a need for a hermetic coupling and method for producing a hermetic seal between a component and a housing that does not suffer from the deficiencies of the prior art.